FIG. 21 shows schematic views of a related-art axial flow fan.
View (a) of FIG. 21 is a perspective view as seen from the upstream side of a flow of a fluid.
View (b) of FIG. 21 is a front view as seen from the downstream side of the flow of the fluid.
View (c) of FIG. 21 is a front view as seen from the upstream side of the flow of the fluid.
View (d) of FIG. 21 is a side view as seen in a direction late al to the axis of rotation of the axial flow fan.
As illustrated in FIG. 21, the related-art axial flow fan includes a plurality of blades 1 disposed along the circumferential surface of a cylindrical boss 2 of the fan. As a rotational force is applied to the boss 2, the blades 1 rotate in a rotational direction 3 to deliver a fluid in a fluid flow direction 5 in which the fluid flows. Each blade 1 has leading and trailing edges curved concavely in the rotational direction. The above-described structure is also disclosed in, for example, Patent Literature 1 and so forth.
In the axial flow fan, when the blades 1 of the axial flow fan rotate, the fluid present between the blades 1 collides with the blade surfaces. The pressure is increased in the surfaces with which the fluid collides, and the fluid is pushed in the axis of rotation direction and moved.
When the blades 1 rotate, the fluid is affected by the centrifugal force and the shape of the blades 1. Thus, as illustrated in FIG. 22, regions of the blade 1, in which the flow velocity in a direction along an axis of rotation 2a is high, are known to gather on the radially outer circumferential side of the blade 1 (for details of actual measured values of the flow velocity distribution in an axial flow fan having a shape illustrated in FIG. 21, see Reito Kucho Gakkai-Shi (Academic Journal of Japan Society of Refrigerating and Air Conditioning Engineers), July 2009, Vol. 84, No. 981, p. 34, FIG. 13 (d)).
Since the axial flow fan is disposed in a bell-mouth 13, the fluid flows in the axis of rotation direction instead of spread in the radial directions.
A pressure loss occurs when the flow velocity distribution, in the axial direction, of the blade 1 of the axial flow fan, as illustrated in FIG. 21, varies in each position. This pressure loss will be described hereinafter.
First, a pressure loss ξ of the fluid is given by:
                    ξ        =                  C          ×                      1            2                    ×          ρ          ×                      v            2                                              [                  Math          .                                          ⁢          1                ]            
where C is the pressure loss coefficient, which is approximately 1 for an open space, ρ is the air density, and v is the flow velocity.
Since the velocity distribution of the fluid varies from one position to another position in the radial direction of the blade, the pressure loss ξ is calculated by dividing the fluid into minute regions.
The square of the flow velocity Vrms of the fluid in one of the minute regions is the sum of the square of an average flow velocity Vave and the square of the standard deviation σ, and accordingly, is given by:Vrms2=Vave2+σ2   [Math. 2]
where Vave is the average flow velocity [m/s] of the fluid, and
σ is the standard deviation [m/s], which is an index representing a deviation from the average flow velocity.
Thus, the pressure loss ξ of the fluid is the sum of squares of the flow velocities in the minute regions and given by Math. 3.
The number of minute regions is the number of equally divided regions (in this case, ten equally divided regions) of the blade 1 in the radial direction.
                                                        ξ              =                            ⁢                              C                ×                                  1                  2                                ×                ρ                ×                                                      (                                                                  v                        1                        2                                            +                                              v                        2                        2                                            +                                              v                        3                        2                                            +                                              …                        ⁢                                                                                                  ⁢                                                  v                          10                          2                                                                                      )                                    10                                                                                                        =                            ⁢                              C                ×                                  1                  2                                ×                ρ                ×                                  1                  10                                ×                                                      ∑                                          l                      =                      1                                        10                                    ⁢                                      v                    i                    2                                                                                                                          =                            ⁢                              C                ×                                  1                  2                                ×                ρ                ×                                  (                                                            v                      ave                      2                                        +                                          σ                      2                                                        )                                                                                        [                  Math          .                                          ⁢          3                ]            
where
ρ is the air density [kg/m3],
v1 to v10 are the local average velocities [m/s] in the case of ten regions equally divided in the radial direction,
Vave is the average flow velocity [m/s], and
σ is the standard deviation [m/s], which is an index representing a deviation from the average flow velocity.
From Maths. 2 and 3. Math. 4 is obtained to calculate the standard deviation σ [m/s], which is an index representing a deviation from the average flow velocity:
                    σ        =                                            1              N                        ⁢                                          ∑                                  i                  =                  1                                N                            ⁢                                                (                                                            v                      l                                        -                                          v                      ave                                                        )                                2                                                                        [                  Math          .                                          ⁢          4                ]            
Math. 3, therefore, reveals that, in order to reduce the pressure loss ξ, σ need only be zero. That is, from the viewpoint of reducing the pressure loss, it is advantageous that the velocity distribution, in the axis of rotation direction, over positions in the radial direction of the blade is ideally flat (uniform flow, that is, the flow velocity is uniform in any position in the radial direction). The flat velocity distribution is achieved by equalizing the velocity distribution by decreasing the high velocity area and increasing the low velocity area.